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Transcript
Conduction in the Human Heart How a Heart Beats • Heart has specific cells that do not contract that specialize in forming and distributing electrical impulses throughout the heart – Called Intrinsic Cardiac Conduction System (individual cells called autorhythmic cells) – Causes regulated depolarization and contraction Sequence of Firing • There are 4 places autorhythmic cells reside in the heart – Sinoatrial (SA) Node – place of original electrical stimulus. Called the pacemaker, generates impulses 75 times every minute. Found in the right atrium – Atrioventricular (AV) Node – After current passes through the SA node, it moves here. Delays impulse to allow atria to complete contraction before ventricles contract. Located in the septum above the tricuspid valve. Firing (cont.) – Right and Left Bundle Branches – Progresses here after AV node. Moves pulse through the septum to the apex of the heart – Purkinje Fibers – Moves here after bundle branches. Causes the rest of ventricular muscles to contract. Heart Readings • We can read the electrical impulses of the heart via an electrocardiogram or EKG. EKG’s consist of three waves, two small waves and a large spike…any more or less than 3 with more or less of an amplitude and your heart has a problem. Waves within an EKG • First small wave is called a P-wave…lasts .08 seconds. Results from depolarization of SA node in atria. – .1 seconds after P wave, atria contracts. • Next comes the QRS complex. This is the large spike we see. The Q is the little dip, R is the large spike, S is the dip at the end. – The QRS complex is the rapid depolarization of the right and left ventricles. Large muscle mass of the ventricles causes the QRS complex to have a much larger amplitude than the P-wave. Waves (cont.) • Lastly we have the T-wave. The T-wave is caused by ventricle repolarization. Repolarization is slow, so it has a much lower amplitude than the QRS. – Apex of T-wave is heart’s absolute refractory period – Last half of the T-wave is heart’s relative refractory period – Heart cannot begin a new impulse until past the refractory periods Intervals • From the beginning of the P-wave to the beginning of the Q in the QRS complex, we have the P-Q interval. This is the time from beginning of atrial contraction to beginning of ventricular contraction (.16 seconds) • S-T interval – The interval from the beginning of S to the beginning of T. Whole myocardium is depolarized Intervals (cont.) • Q-T interval – From the beginning of Q to end of T (Q-T interval includes the S-T interval). Marks time from ventricular depolarization to repolarization. (Something to know for your quiz: this is the period where the ventricles contract.) Lasts .38 seconds Video explaining an EKG